1. Field of The Invention
The present invention relates generally to a system for controlling damping force characteristics of respective shock absorbers of a vehicle, each shock absorber (also called a damper, but hereinafter referred to as the shock absorber) being interposed between an unsprung mass and sprung mass of the vehicle so as to provide an optimum damping force therefrom. The shock absorber is provided with changing means for changing the damping coefficient thereof at multiple stages in response to a drive signal.
2. Description of The Background Art
A Japanese Patent Application First Publication No. Showa 61-163011 exemplifies a previously proposed damping characteristic controlling system for a vehicle.
In the disclosed Japanese Patent Application First Publication, a relative velocity between sprung mass (vehicle body) and unsprung mass (tire wheel) is detected. When both velocities have mutually the same sign, the characteristic of the damping force for one shock absorber is set to have a hard characteristic. When both velocities have the different signs, the characteristic of the damping force for the one shock absorber is set to have a soft characteristic. The setting operations are based on the damping force characteristic control called "Sky Hook" theory and carried out independently for each of the four tire wheels.
Thus, an application energy to be transmitted to the vehicle body can be reduced and, at the same time, an energy to damp the vibration can be increased. Consequently, vibrations occurring on the vehicle body can be suppressed and a highly improved vehicle comfort can be achieved. In addition, such phenomena as squat and/or dive can be suppressed and steering stability of the vehicle can be improved.
However, since when, in the suspension control system disclosed in the above-identified Japanese Patent Application First Publication, a control gain is determined such that the vibration energy transmission from a road surface to the vehicle body (sprung mass) is suppressed, while assuring the characteristic to control vibration damping, a rolling occurs on the sprung mass due to the application of inertia force to the sprung mass during the vehicular steering operation and a control force becomes insufficient by the inertia force so that a sufficient effect of damping the vibration on the sprung mass cannot be achieved.